Oceans soak up about a quarter of the carbon dioxide released into the atmosphere each year and as CO2 levels in the atmosphere increase from burning fossil fuels, so do ocean levels, making seas more acidic.

Ocean acidification is one of the effects of climate change and threatens coral reefs, marine ecosystems and wildlife.

The shell of the pteropod sea snail in the Southern Ocean was severely dissolved by more acidic surface water, the researchers from the British Antarctic Survey, Royal Netherlands Institute for Sea Research, the US National Oceanic and Atmospheric Administration (NOAA) and other institutions found.

And although the snails did not necessarily die, it increased their vulnerability to predators and infection which could affect other parts of the food chain.

"The corrosive properties of the water caused shells of live animals to be severely dissolved and this demonstrates how vulnerable pteropods are," says lead author Nina Bednaršek, from the NOAA.

"Ocean acidification, resulting from the addition of human-induced carbon dioxide, contributed to this dissolution."

The sea snails are an important source of food for fish and birds as well as an indicator of marine ecosystem health.

But until now, there has been little evidence of the impact of ocean acidification on such live organisms in their natural environment and the study supports predictions that acidification could have a significant effect on marine ecosystems.

Rising levels, more mixing

The researchers examined surface water, where wind causes cold water to be pushed up from deeper water, because it is usually more corrosive to a particular type of calcium carbonate which the sea snails use to build and maintain their shells.

"We know that the seawater becomes more corrosive ... below a certain depth which occurs at around 1000 metres. However, at one of our sampling sites, we discovered that this point was reached at 200 metres depth. Marine snails - pteropods - live in this top layer of the ocean," says Bednaršek.

Climate models forecast more intense winds in the Southern Ocean this century if CO2 continues to increase, which will make the mixing of deep water with more acidic surface waters more frequent, the study's author say.

This will make calcium carbonate reach the upper surface layers of the Southern Ocean by 2050 in winter and by 2100 all year round, says the co-author Dorothee Bakker, a research officer at the University of East Anglia.

Since the start of the industrial revolution, the acidity of surface ocean waters has increased by 30 per cent, according to NOAA research.

If CO2 levels continue to rise in the future, surface waters could be almost 150 per cent more acidic by the end of this century, which has not been experienced for more than 20 million years.